Heating systems that rely on the circulation of hot water or steam from a boiler through heat exchange means are generally known as hydronic type heating systems. Hot water hydronic heating systems typically have been operated with a circulating pump energized concurrently with a fuel burner which heats water in the boiler. With this type of a hydronic system operation, the circulator pump initially starts circulating water that is relatively cool through heat exchangers in the boiler and in the house (radiators) and back to the boiler. This tends to reduce the boiler temperature to a point where condensation of the water in the combustion products occurs on the outside of the boiler heat exchanger and this in turn leads to rusting of the heat exchanger. This type of an operation shortens the life of the boiler heat exchanger and is undesirable.
One way to avoid this type of corrosive action is to provide the boiler with a sensor that controls the circulator by temperature. The burner is put into operation and the circulator pump is held out of operation until some predetermined temperature has been reached that is considered high enough to avoid condensation on the outside of the boiler heat exchanger. Typically, this is a temperature of 105 degrees Fahrenheit. This type of a system, if reliable, would generally solve the rusting problem.
Unfortunately, this type of a system has the potential to be unreliable. The temperature sensor may fail to act properly, and the boiler can be operated indefinitely without the temperature rise being sensed or properly acted upon. By merely sensing the boiler water temperature and operating the circulator pump based on a fixed temperature, many operating problems are undetected and the system can be either inefficiently or unsafely operated.